1. Field of the Invention
The invention relates to a wavelength stabilization technique, and particularly for wavelength chirp compensation of initial pulses in bursts for an excimer or molecular fluorine laser operating in burst mode.
2. Discussion of the Related Art
Excimer and molecular fluorine lasers may be typically operated in burst mode. This means that the laser generates xe2x80x9cburstsxe2x80x9d of pulses, such as 100 to 500 pulses at a constant repetition rate, followed by a burst break or pause of from a few milliseconds up to a few seconds while the stepper/scanner does some wafer positioning. During this pause, the laser may be shifted to low duty cycle such as 50 Hz from 2-4 kHz or more during the burst, or there may be no pulses generated during the pause. A burst break may be a short burst break such as may occur when the beam spot is moved to a different location on a same wafer, or may be a long burst break such as would occur when the stepper/scanner changes between wafers.
When an excimer or molecular fluorine laser is operated in burst mode, the first few pulses of each burst will have a varied wavelength from later pulses if left uncompensated. This variance at the beginning of bursts, hereinafter referred to as xe2x80x9cwavelength chirp,xe2x80x9d may occur, e.g., due to cooling of optics and corresponding refractive index changes in the optics that occur during burst pauses. It is desired to compensate wavelength chirp in order to achieve a constant wavelength of laser pulses throughout bursts.
The problem that the first few pulses after a burst break (at the beginning of a burst) have a different wavelength than the pulses in the middle or at the end of a burst, can be generally understood from the qualitative plots of FIGS. 1c and 2c. FIG. 1c illustrates how the wavelength for an ArF laser having a beam expander/grating line-narrowing configuration is far below a target wavelength at the beginnings of bursts after burst pauses when uncompensated. FIG. 2c illustrates how the wavelength for an KrF laser having a beam expander/grating/etalon line-narrowing configuration is far above a target wavelength at the beginnings of bursts after burst pauses when uncompensated. In the sketches of each of FIGS. 1c and 2c, the first few pulses have a varied wavelength from a target wavelength, and then the wavelength deviation from the target wavelength reduces rapidly until the wavelength reaches approximately the target level. The variance at the beginning of the bursts shown in FIGS. 1c and 2c illustrates wavelength chirp.
In order to provide a constant wavelength through burst including the first few pulses (which is desired during laser operation), the grating and or etalon or other tuning means of the laser wavelength tuning module may be adjusted to a different position at the beginning of the bursts as after the initial pulses. The exact behavior of the wavelength is affected by various parameters in a way that is difficult to predict.
A learning algorithm is described in U.S. Pat. No. 6,078,599 for correcting wavelength chirp. The algorithm operates the laser and measures the wavelengths of pulses in bursts. Then, in future bursts, optical components are tuned to provide a same wavelength at the beginnings of bursts as after the chirp affects the pulses later in the burst, based on what was learned from the earlier measurements on how much the wavelength was deviated. It is difficult, however, to accurately measure wavelengths of individual pulses at the beginnings of bursts, and to accurately predict a wavelength compensation needed for bursts following burst pauses of varying duration. In addition, it is undesirable to have a wavelength compensation technique wherein initial bursts after starting operation of the laser system are used for learning, and having to wait for subsequent bursts in order to apply the wavelength chirp information learned from measurements of the initial bursts. It is desired to have a reliable technique for adjusting the wavelengths of pulses at the beginnings of bursts to a target value.
It is desired to have a reliable technique for compensating wavelength chirp in excimer and molecular fluorine laser systems for adjusting the wavelengths of pulses at the beginnings of bursts to a target value. In addition to that whch has been described above, there are short-term effects and long-term effects that influence the behavior associated with the wavelengths of pulses during bursts and from burst to burst. Short-term effects may last for only a few seconds or less. Long-term effects include gas aging (several days), tube aging (several months) and maybe optical effects (years). These effects may be taken into account by changing controller parameters. The parameter adaptation may be advantageously performed automatically.
The wavelength chirp behavior changes depending on the length of the burst break, the repetition rate of the laser, the wavelengths and/or energies of the most recent pulses and other effects. It is more difficult to control the wavelengths of the first pulses in a burst than it is to keep the wavelength constant for pulses at the middle and end of a burst because the condition of the optics and the laser gas mixture, e.g., do not change as rapidly with time over the duration of the burst as they do during a burst pause. It is thus desired to have a wavelength chirp control algorithm that produces wavelength stability of pulses to a same target wavelength at the beginning of a burst as throughout the entirety of the burst.
In view of the above, a wavelength chirp compensation method for an excimer or molecular fluorine laser system operating in burst mode, comprising pre-programming into a computer of the laser system resonator tuning optic adjustments for making the adjustments during pauses between bursts to compensate wavelength chirp at beginnings of succeeding bursts.
In particular, a first wavelength chirp compensation method includes operating a gas discharge laser system at an operating duty cycle, e.g., 2-4 kHz, such the laser is operating in a condition such as during a burst when the laser is operated in burst mode for processing structures on a wafer. The laser is then switched to a nominal duty cycle, e.g., 5-50 Hz just sufficient to enable wavelength measurements to be made, and low enough that laser operating conditions change as they would during a burst pause for a laser operating in burst mode. The wavelength is measured for a period of time without adjusting resonator optics to adjust the wavelength, i.e., any wavelength control algorithm available with the laser system is switched off. Resonator optics adjustments are calculated based on the measurements over this period of time and programmed into a computer of the laser system. When the laser is later operated in burst mode, an optic or optics of the laser resonator is/are adjusted throughout burst pauses according to the programmed adjustments until the beginnings of next bursts, wherein the optic or optics is/are ready to produce pulses at the target wavelength when the next burst begins. The optic or optics is/are returned to standard operating position after the chirp period, such that wavelength chirp is efficiently compensated.
In addition, a second wavelength chirp compensation method includes operating a gas discharge laser system at an operating duty cycle, e.g., 2-4 kHz or more, such the laser is operating in a condition such as during a burst when the laser is operated in burst mode for processing structures on a wafer. The laser is then switched to a nominal duty cycle, e.g., 5-50 Hz, just sufficient to enable wavelength measurements to be made, and low enough that laser operating conditions change as they would during a burst pause for a laser operating in burst mode. A wavelength control algorithm for adjusting the wavelength in a feedback loop between a wavelength measurement device and resonator optics is used to control the wavelength to a target wavelength. The positions of the optic or optics of the resonator are recorded for a period of time and programmed into a computer of the laser system. When the laser is later operated in burst mode, the optic or optics of the laser resonator is/are adjusted throughout burst pauses according to the programming, i.e., just as they were adjusted when the control algorithm was used during the nominal duty cycle operation of the laser, until the beginnings of next bursts, wherein the optic or optics is/are ready to produce pulses at the target wavelength when the next burst begins. The optic or optics is/are returned to standard operating position after the chirp period, such that wavelength chirp is efficiently compensated.
According to either of the first and second methods summarized above, the optic or optics of the laser resonator are stepped to adjusted positions throughout burst pauses. Longer burst pauses will result in the optics being moved further from standard position than shorter pauses, and typically higher wavelength chirps after the longer burst pauses are compensated, as well as those after shorter burst pauses, by appropriate amounts of movement of the resonator optic or optics during the burst pauses. Advantageously, the pause length need not even be known in advance, such that whenever the initial pulses begin for the next burst, the resonator optics are already adjusted in position to compensate wavelength chirp according to burst pause duration. In addition, the technique may be used for initial bursts of the laser, because no learning algorithm is required for computing resonator optics adjustments needed for burst pauses of any length, not to mention burst pauses of many varied lengths.